1. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to adaptive antenna systems.
2. Description of the Related Art
With the advent and proliferation of digital communication systems, the need for high capacity, high performance systems continues to accelerate. These needs have prompted a strong interest in the development of efficient antenna systems for use at a base station. Efficient antenna systems can increase the capacity and performance of existing digital communications systems without modification of the standardized wireless link protocols.
FIG. 1 shows a typical base station 10 and its corresponding coverage area. The coverage area of the base station 10 corresponds to the geographical region over which the base station 10 is capable of servicing a remote unit. For example, within the coverage area of the base station 10, a series of remote units 12A-12N are shown. The base station 10 is sectored in that it provides three distinct coverage areas 14A, 14B and 14C in a manner typical of modem base stations. In general, a base station comprises three or more sectors dividing the coverage area into 120xc2x0 or smaller sections to provide a 360xc2x0 azimuth field. The use of sectors improves the overall performance and capacity of the base station.
Each sector 14A-14C has a separate antenna system. The use of separate systems decreases the interference between remote units located in different sector coverage areas. For example, the remote unit 12C is within the coverage area 14B and, therefore, provides very little interference to the remote unit 12N located within the coverage area 14C. In contrast, remote units 12A and 12B are each located within the coverage area 14A, therefore, their signals interfere with one another to some extent at the base station 10.
To reduce the interference created by remote units operating within a common coverage area, a variety of multiple access schemes have been developed. For example, code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA) or frequency hopping can be used to reduce the interference within a sector. In each of these types of systems, the use of multibeam antenna systems to further sectorize the base station coverage area further reduces co-channel interference and increases the capacity of the system.
For example, to further reduce the interference between remote units within a sector, an antenna array can be used to divide a typical 120xc2x0 base station sector coverage area into smaller segments called xe2x80x9cbeamsxe2x80x9d. FIGS. 2A and 2B are graphs showing a typical narrow-beam coverage area pattern in polar and rectangular format, respectively. As shown in FIGS. 2A and 2B, in addition to a narrow main beam 20A, multiple sidelobes 20B-20E are also present. In general, the amplitude of the sidelobes 20B-20E are lower than the main lobe 20A. For example, in the embodiment illustrated in FIGS. 2A2B, each sidelobe 20B-20E is at least 30 decibels (dB) down from the main lobe 20A.
FIGS. 3A and 3B show a top view and a side view of an antenna array capable of producing the coverage area pattern shown in FIGS. 2A and 2B. Each of the three antenna arrays 24A-24C is made up of eight array elements 26A-26H. Together the three antenna arrays 24A-24C provide a full 360xc2x0 coverage area. In FIG. 3B, the eight array elements 26A-26H have a nominal one-half wavelength spacing. FIG. 3C is a block diagram showing additional circuitry coupled to the antenna array 24A which make up a beamformer capable of producing the coverage area pattern shown in FIGS. 2A and 2B. The output of each array element 26A-26H is coupled to a weighting block 28A-28H, respectively. The weighting blocks 28A-28H provide amplitude tapering and phase shifting, thus, effectively multiplying the incoming signals by a complex set of weights, {Wm, m=1 . . . 8}. (Through out this text, complex functions and numbers are denoted by underscored text.) The outputs of the weighting blocks 28A-28H are summed in a summer 30. Weighting the output of each array element 26A-26H by the weighting blocks 28A-28H controls the gain at the peak of the beam, the width of the beam and the relative gain of the sidelobes.
Each array element 26A-26H within the antenna array 24A ideally has an identical pattern gain and shape over the field of view of the array. This pattern, called the element factor, typically varies as the function of the angle from the normal to the array face. In typical systems, the antenna array comprises 8 or 16 array elements (i.e., m=8 or m=16) and associated weighting blocks. The weighting blocks shown in FIG. 3C are sufficient to create one narrow beam such as shown in FIG. 2A. To create additional beams, additional weighting blocks and summers must be used.
Referring again to the example of FIG. 2A, if a remote unit 22A is located within the main lobe 20A and a remote unit 22B is located within the sidelobe 20, the base station receives the signal energy transmitted by both the remote unit 22A and 22B. Although the signal from the remote unit 22B is reduced by the gain of the sidelobe relative to the main beam, the signal from the remote unit 22B may still cause significant interference with the signal from the remote unit 22A.
In the prior art, adaptive antenna techniques have been used to change the coverage area pattern when the remote unit signal within a sidelobe is interfering with the signals in the main beam. These adaptive antenna techniques detect the presence of an interfering signal and modify the coverage area pattern generated by the antenna beamformer to further suppress the interfering signals in the sidelobes. For example, in the situation shown in FIG. 2A, it would be advantageous to decrease the size of or place a null in the sidelobe 20E so that the effects of signal from the remote unit 22B on the signal from remote unit 22A may be reduced. Prior art has proposed many of these xe2x80x9csmart antenna arrayxe2x80x9d designs to achieve this purpose, but in general, their complexity makes their implementation costly and limits their use in standard terrestrial wireless systems.
In the case shown in FIG. 2A, a null can be placed within the sidelobe 20E to decrease the effects of the signal from the remote unit 22B on the system. However, placement of a null within a sidelobe produces a corresponding increase in sidelobe-gain at some other location as illustrated in FIG. 2C. In FIG. 2C, nulls have been place at approximately xe2x88x9260,xe2x88x9240,20,38 and 60 degrees from boresight. Notice that the sidelobe having a peak at approximately 28 degrees from boresight has a maximum gain that is greater than xe2x88x9220 dB with respect to the gain of the main lobe. In fact, it is possible for the gain of a sidelobe to exceed the gain of the main lobe if certain weighting parameters are selected.
FIG. 4 is a block diagram showing an adaptive null steering system which is also known in the art as a coherent sidelobe cancellation antenna system. The system includes an antenna array 40 which operates in a similar manner to the system shown in FIG. 3C. For example, the antenna array 40 can be configured to produce a standard narrow beam such as the antenna pattern shown in FIG. 2B. The antenna pattern includes the sidelobes 20B-20C as shown. In addition, the antenna system in FIG. 4 comprises two auxiliary antennas 42A and 42B. The antennas 42A and 42B are coupled to complex weighting blocks 44A and 44B, respectively. The values D1 and D2 within the elements 44A and 44B, respectively, are complex weights which can be set to form an auxiliary antenna pattern. For example, an antenna pattern 82 in FIG. 5 represents an antenna pattern for the auxiliary antennas 42A and 42B. Note that the antenna pattern 82 forms a beam which encompasses the sidelobe area corresponding to the antenna pattern shown in FIG. 2B and has a null in the direction of the main beam. A broader null in the direction of the main beam can be developed with the use of additional auxiliary antennas such as such shown in FIG. 5 as an antenna pattern 84 which is created using four auxiliary elements.
The output of the complex weights 44A and 44B are coupled to a summer 46 which produces a combined output. The combined output is input into a complex weighting block 48 which applies complex weight xcex2. The output of the complex weighting block 48 is coupled to a summer 50 which sums the output of the antenna array 40 with the output of the complex weighting block 48.
When a signal is received through a sidelobe of the antenna pattern, the same signal is also received through the auxiliary antennas 42A and 42B. However, the phase and amplitude of the signal received through the antenna array 40 and the auxiliary antennas 42A and 42B is different at the input to the summer 50. If the amplitude and phase is properly adjusted, the signal energy which has been received through the auxiliary array can be coherently subtracted from the signal energy received through the sidelobe of the main beam. In order to adjust the complex weight xcex21, the output of the summer 50 is cross-correlated with the output of the summer 46 using coherent (phase sensitive) detection by a cross-correlator 52. If a signal is present both at the output of summer 50 and the summer 46, it is detected by the cross-correlator 52. By integrating the output of the cross-correlator 52, an error signal is generated which can be used to adjust the value of the complex weight A to reduce the energy received through the sidelobes at the output of the summer 50 according to well known techniques, such as Widrow""s least mean squared (LMS) algorithm as described in B. Widrow, et. all, Adaptive Antenna Systems, Proceedings of the IEEE, Vol. 55, No. 12, December 1967, pp. 2143-2159. As a result, a null in the direction of the undesired signal is created in the combined pattern of the main and auxiliary antenna beams.
As noted above, as the adaptation algorithm adjusts the gain of the sidelobes to steer a null in the direction of one or more interfering signal, the gain of other sidelobes may increase. If the gain of these sidelobes is allowed to increase, two undesirable results can occur. First, the total interference level is increased by additional interference and noise received through the undesirably high sidelobes. Second, the probability that a new interfering signal source will appear within the undesirably high sidelobe and cause interference until the adaptation algorithm can react to squelch it also increases.
Therefore, there is the need in the art for a smart antenna array with high performance yet which is less complex and more modular than existing systems. In addition, there is a need in the art for a method of maintaining a acceptable sidelobe level while concurrently adapting to suppress high level interference within the sidelobe region.
An antenna beam is adapted to current operating conditions by determining a maximum gain value of a sidelobe region of the adaptive antenna pattern and, also, determining a corresponding angle at which the maximum gain value is achieved. Next, a min-max gradient of the adaptive antenna pattern at the corresponding angle is determined. A next value of a first partial weighting value is then determined according to a current value of the first weighting value, a first predetermined step size, a first predetermined decay constant and the min-max gradient. The first partial weighting value is used to determine the adaptive pattern of the antenna beam. The next value of the first partial weighting value is determined so that it tends to limit the maximum gain value within the sidelobe region. For example, the first partial weighting value can tend to maintain a relatively uniform gain within the sidelobe region.
In addition, a null-steering gradient of an adaptation error is determined based upon a set of cross-correlation measurement samples reflecting the current operating conditions. A next value of a second partial weighting value is determined according to a current value of the second partial weighting value, a second predetermined step size, a second predetermined decay constant and the null-steering gradient. The second partial weighting value is also used to determine the adaptive pattern of the antenna beam. The next value of the second partial weighting value is determined so that it tends to steer a null in the direction of an interfering signal received through the sidelobe region.
Based upon the next value of the first partial weighting value and the next value of the second partial weighting value, a beamforming weight is updated. The beam forming weight is used by an antenna array to create the antenna beam. In this way, the antenna beam is adapts to current operating conditions without adapting to a pattern with excessively high sidelobe regions.
The maximum gain value of the adaptive antenna pattern can be calculated open loop. For example, the adaptive antenna pattern can be determined according to:                     E        _            k        ⁡          (                        θ          k                ,                  Φ          k                    )        =            ∑              m        -        1            M        ⁢                                        W            _                                k            ,            m                          ⁡                  (          i          )                    ⁢              ⅇ                  j          ⁡                      [                                          m                ⁡                                  (                                                            2                      ⁢                                              xe2x80x83                                            ⁢                      π                                        λ                                    )                                            ·                              d                ⁡                                  (                                                            sin                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        k                                                              -                                          sin                      ⁢                                              xe2x80x83                                            ⁢                                              Φ                        k                                                                              )                                                      ]                              
wherein:
Ek(xcex8k, "PHgr"k) represents a gain value of the adaptive antenna pattern at an evaluation angle, xcex8k;
d is the distance between antenna elements of an antenna array producing the antenna beam in meters;
xcex is the wave length of a receive signal in meters.
"PHgr"k is the center angle of a main beam of the adaptive antenna pattern with respect to boresight; and
xcex8k is the evaluation angle at which the gain value is evaluated.
The min-max gradient can be determined according to:                     Γ        _            m        ⁡          (                        i          -          1                ,                  θ                      k            -            Max                          ,                  Φ          k                    )        =            "LeftBracketingBar"                                    E            _                    k                ⁡                  (                                    θ                              k                -                Max                                      ,                          Φ              k                                )                    "RightBracketingBar"        ⁢          ⅇ              j        ⁡                  [                      m            ·                          (                                                2                  ⁢                                      xe2x80x83                                    ⁢                  π                                λ                            )                        ·                          d              ⁡                              (                                                      sin                    ⁢                                          xe2x80x83                                        ⁢                                          θ                                              k                        -                        Max                                                                              -                                      sin                    ⁢                                          xe2x80x83                                        ⁢                                          Φ                      k                                                                      )                                              ]                    
wherein:
xcex93m(ixe2x88x921, xcex8k-Max) is the min-max gradient;
xcex8k-Maxis approximately the corresponding angle; and
Ek(xcex8k-Max, "PHgr"k) is the maximum gain value of the adaptive antenna pattern at the corresponding angle, xcex8k-Max.
Using these values, the next value of the first partial weighting value can be determined according to:
Ak,m(i)=xcfx81Axc2x7Ak,m(ixe2x88x921)xe2x88x92"ugr"Axc2x7xcex93k,m(ixe2x88x921, xcex8k-Max, "PHgr"k)/|xcex93k,m(ixe2x88x921, xcex8k-Max, "PHgr"k)|
wherein:
Ak,m(i) is the next value of the first partial weighting factor;
Ak,m(ixe2x88x921) is the current value of the first partial weighting factor;
xcfx81A is the first predetermined decay constant; and
"ugr"A is the first predetermined step size.
The null-steering gradient of the adaptation error can be determined by measuring a level of current energy received through the antenna beam and mathematically applying a transfer characteristic of a phantom auxiliary beam. For example, the null-steering gradient of the adaptation error can be determined according to:                     Λ        _                    k        ,        q              ⁡          (      i      )        =            ∑              m        =        q                    P        +        q              ⁢                            D          _                          k          ,          m                    ⁢                                    C            _                                k            ,            m                          ⁡                  (          i          )                    
wherein:
xcex9k,q(i) is the null-steering gradient of the adaptation error for a qth phantom auxiliary beam for the antenna beam k;
Ck,m(i) is a cross-correlation measurement sample set of signal energy received each array element, m, of an antenna array cross-correlated with energy in a compensated output of the antenna beam;
Dk,p(i) is a complex weight which determines the contribution of a pth array element to the qth phantom auxiliary beam for the antenna beam;
Q is a total number of phantom auxiliary beams; and.
P is a total number of array elements used to create each phantom auxiliary beam.
The adaptation method just described can be used with a variety of antenna configurations. For example, one advantageous antenna configuration which can be used with the method is one in which a modular set of modules are concatenated together. Such an adaptive antenna system includes a plurality of array element modules, each array element module has an antenna element. The antenna element makes up one component of an antenna array. A programmable delay element has an input coupled to an output of the antenna element. The programmable delay element is configured to produce a delayed output.
Each array element module also has a weighting circuit. The weighting circuit has an antenna sample input coupled to the delayed output of the programmable delay element. The weighting circuit also has a composite signal input and a composite signal output. The weighting circuit is coupled to a previous weighting circuit within a previous array element module in a concatenated manner such that the composite signal output from the previous weighting circuit is coupled to the composite signal input of the weighting circuit. The weighting circuit is configured to apply a complex weight to samples received from the antenna sample input to produce weighted antenna samples. The weighting circuit is also configured add the weighted antenna samples to samples received from the composite signal input and to provide a resultant signal to the composite signal output.
The array element module also has a second delay element having an input coupled to the output of the antenna element and having a delayed output. Finally, the array element module has a cross-correlation measurement circuit. The cross-correlation measurement circuit has an antenna sample input coupled to the delayed output of the second delay element. The cross-correlation measurement circuit also has an adaptive error input and a cross-correlation measurement output. The cross-correlation measurement circuit is configured to cross-correlate samples received from the antenna sample input with samples received from the adaptive error input to provide cross-correlation measurement samples to the cross-correlation measurement output.
The plurality of array element modules are controlled by an adaptation controller. The adaptation controller has a controller input coupled to the cross-correlation measurement output of the cross-correlation measurement circuit within each of the plurality of array element modules. The adaptation controller also has a weighting output. The adaptation controller is configured to determine the complex weight to provide the weighting circuit within each of the plurality of array element modules. The adaptation controller determines the complex weights based upon the cross-correlation samples at the controller input.
In one embodiment, the cross-correlation measurement circuit further has a delayed adaptive error output configured to provide a delayed version of the samples received from the adaptive error input. The cross-correlation measurement circuit is coupled to a previous cross-correlation measurement circuit within the previous array element module in a concatenated manner such that the delayed adaptive error output from the previous cross-correlation measurement circuit is coupled to the adaptive error input of the cross-correlation measurement circuit. The composite signal output of a last weighting circuit within a last one of the plurality of array element modules can be coupled to the adaptive error input of a first cross-correlation measurement circuit within a first one of the plurality of array element module, such as via a channel filter.
In another embodiment, each of the plurality of array element modules comprises a plurality of the weighting circuits and a plurality of the cross-correlation measurement circuits, each pair of which corresponds to one of K antenna beams. In yet another embodiment, the adaptation controller is configured to determine the complex weight using a min-max adaptation algorithm which tends to limit a maximum gain value within a sidelobe region the antenna beam and a null steering adaptation algorithm which tends to steer a null in the direction of an interfering signal received through the sidelobe region.